#define _CRT_SECURE_NO_WARNINGS  1
#include <stdio.h>

int main() {
    long long int n = 0;
    long long int avg = 0;
    long long int r = 0;
    while (scanf("%lld%lld%lld", &n, &r, &avg) != EOF)
    {
        long long int time = 0;
        long long int arr[1000][2];
        long long int i = 0;
        long long int score1 = 0;
        for (i = 0; i < n; i++)
        {
            scanf("%lld %lld", &arr[i][0], &arr[i][1]);
            score1 += arr[i][0];
        }
        if (score1 >= (avg * n))
        {
            time = 0;
        }
        else
        {
            long long int score2 = (avg * n) - score1;
            long long int arr1[1000];
            for (i = 0; i < n; i++)
            {
                arr1[i] = arr[i][0];
            }
            while (score2 > 0)
            {
                long long int min = 100000;
                long long int p = 0;
                for (i = 0; i < n; i++)
                {
                    if ((arr[i][1] < min) && arr1[i] != r)
                    {
                        min = arr[i][1];
                        p = i;
                    }
                }
                if (score2 > (r - arr1[p]))
                {
                    score2 -= (r - arr1[p]);
                    time += min * (r - arr1[p]);
                }
                else
                {
                    time += (min * score2);
                    score2 = 0;
                }
                arr1[p] = r;
            }
        }
        printf("%lld\n", time);
    }
    return 0;
}